1. Field of the Invention
The present invention relates to a spread illuminating apparatus suitable for a display device to operate with outside light, especially a liquid crystal display heavily used in a personal computer, a mobile communications device, and the like.
2. Description of the Related Art
A liquid crystal display (hereinafter referred to as LCD) featuring low profile, small occupying volume and light weight is used in many electric devices, such as a personal computer (hereinafter referred to as PC) and a mobile phone, and is expected to continue to be increasingly demanded.
A liquid crystal of the LCD does not emit light by itself, so the LCD requires an illuminating means to irradiate the liquid crystal when used in a place where sunlight or room lighting is not fully available. Since it is very inconvenient if an illuminating means is arranged discrete from an electric device equipped with an LCD, the illuminating means is usually arranged with the LCD as a unit. And, the illuminating means is required to be low in profile and small in power consumption, when used for a PC, in particular a notebook-size PC, and a mobile phone. These requirements are filled by a spread illuminating apparatus of side light type.
FIG. 11 is a perspective exploded view of a conventional spread illuminating apparatus of this type disclosed in Japanese Patent Laid-open No. 2000-11723. As shown in FIG. 11, a spread illuminating apparatus 1 is disposed over a front surface (upper side in the figure) F of a reflection-type liquid crystal element L, which is a main body of a reflection-type LCD. The spread illuminating apparatus 1 comprises a light conductive plate 2 made of a light-permissible material, a light conductive bar 7, and spot-like light sources 9, 9 such as light emitting diodes (LEDs).
The light conductive bar 7 is disposed with its side surface 13 in contact with an end surface 3 of the light conductive plate 2, and has the spot-like light sources 9, 9 arranged on its both end surfaces 8, 8, respectively. An optical path conversion means 11 adapted to direct lights from the spot-like light sources 9, 9 toward the end surface 3 of the light conductive plate 2 is formed on a side surface 14 of the light conductive bar 7 opposite to the side surface 13. The optical path conversion means comprises, for example, a plurality of grooves 15 substantially triangular in section and a plurality of flat portions 16 each present between adjacent grooves 15, 15.
In the spread illuminating apparatus 1 thus structured, lights from the spot-like light sources 9, 9 enter the light conductive bar 7, have their optical paths changed by the optical conversion means 11 formed on the side surface 14 of the light conductive bar 7, and are directed toward and introduced into the light conductive plate 2 through the end surface 3 of the light conductive plate 2. The lights introduced into the light conductive plate 2 travel toward an end surface 10 thereof opposite to the end surface 3 while repeating reflection and refraction therein, exit out meanwhile through a bottom surface thereof, and illuminate the reflection-type liquid crystal element L disposed close to a bottom surface 5 of the light conductive plate 2, whereby the reflection-type liquid crystal element L performs an emission (indirect emission) display.
The conventional illuminating apparatus 11 has the problem that the light exiting out from the side surface 13 of the light conductive bar 7 toward the end surface 3 of the light conductive plate 2 is not uniform in intensity along the length of the side surface 13.
FIG. 12 illustrates schematically the problem. Attention is now called to lights r3 and r4 which exit out orthogonally from the side surface 13 of the light conductive bar 7 in contact with the end surface 3 of the light conductive plate 2 and which are incident orthogonally on the end surface 3. Lights r1 and r2, from which the lights r3 and r4 originate, respectively, are parallel to each other if the grooves 15 (not shown in FIG. 12) formed on the light conductive bar 7 have a constant angle. This means that lights, from which the lights r1 and r2 originate, respectively, have respective different incidence points on the end surface 8 of the light conductive bar 7.
The intensity of the incident light on the end surface 8 of the light conductive bar 7 differs from point to point. Specifically, the intensity is high at a point in contact with the central portion of the spot-like light source 9, and low at a point in contact with the end portion thereof or not in contact with any portion hereof. Thus, the light r1 has a high intensity and the light r2 has a low intensity. Accordingly, as to the lights r3 and r4 exiting out from the side surface 13 of the light conductive bar 7, the light r3 has a high intensity and the light r4 has a low intensity, causing non-uniformity in the intensity of incident light on the end surface 3 of the light conductive plate 2. Further, lights r1′ and r3′ have a high intensity, and the lights r2′ and r4′ have a low intensity. FIG. 12 shows only the left end portion of the light conductive bar 7, but this happens also at the right end portion thereof.
In a spread illuminating apparatus shown in FIG. 13, the brightness was measured at the section taken along A-A′ on the light conductive plate 2, and the brightness distribution α in FIG. 14 was obtained. As shown in FIG. 14, in the conventional apparatus, the light is not uniformly distributed (the distribution is not even) along the length of the light conductive bar 7, causing a problem that the light distribution at the light conductive plate 2 is not uniform.